Presence Of Gamma-secretase Inhibitors Research Articles

Involvement of Notch signaling pathway in regulating IL-12 expression via c-Rel in activated macrophages

Macrophages play an important role both in innate and adaptive immune responses. Treatment with interferon (IFN) γ together with lipopolysaccharide (LPS) activates pro-inflammatory macrophages which secrete various pro-inflammatory cytokines including IL-12. IL-12 promotes a Th1 type immune response by directly controlling the differentiation of CD4+ T helper 1 cells. Activation of Notch signaling pathway was reported in activated macrophages but the involvement of this signaling pathway in IL-12 expression has not been documented. In this study, we investigated the role of Notch signaling in regulating expression of the IL-12/IL-23 subunit, IL-12p40. Using a gamma-secretase inhibitor (GSI) to inhibit Notch signaling, we observed a profound decrease in il12p40 mRNA levels and IL-12p70 secretion upon IFNγ/LPS stimulation. On the other hand, overexpression of activated form of Notch1 in activated RAW264.7 macrophage-like cell lines significantly increased the level of il12p40 mRNA. GSI treatment did not affect the expression of irf5, a master regulator of il12p40 transcription in macrophages. Detailed analysis of the signaling cascades that were affected by this inhibition showed that c-Rel nuclear translocation was inhibited and Erk1/2 activation was compromised by GSI treatment. Addition of exogenous tumor necrosis factor (TNF) α only partially rescued the expression of il12p40 in the presence of GSI. Unexpectedly, inhibition of Notch signaling using a dominant negative (DN) Mastermind-like (MAML) transcription co-activator, did not affect c-Rel nuclear localization upon activation or il12p40 mRNA levels, suggesting that the transcriptional activity of Notch signaling is dispensable for the activation of c-Rel. These results strongly suggest that Notch signaling in activated macrophages is involved in regulating the expression of il12p40 directly via c-Rel and indirectly via TNFα production.

Activation of Notch Signaling Is An Early Event in the Development of PML-Rara-Induced Acute Promyelocytic Leukemia (APL)

Abstract 2468Acute promyelocytic leukemia (APL, FAB M3) is characterized by the presence of the t(15;17) translocation, resulting in production of the oncogenic transcription factor PML-RARA. We previously reported a gene expression signature of 1121 annotated genes that distinguish APL from the other FAB subtypes and normal promyelocytes (Payton et al, JCI 2009). Within this signature, we identified the Notch ligand JAG1 as being overexpressed in APL samples compared to both other AML subtypes (APL/AML=4.2, FDR<0.5%) and normal promyelocytes (APL/pros=5.66, FDR<0.5%). We then measured JAG1 expression in mouse and cellular models of APL. High levels of JAG1 mRNA and protein were found in both zinc treated PR9 cells (which contain a zinc inducible PML-RARA cassette), and in NB4 cells, a human APL cell line. APL cells derived from the mCG-PML-RARA knockin mouse overexpressed Jag1 mRNA compared to wildtype promyelocytes (APL/pros=10.4, p<0.01). Additionally, Jag1 protein was readily detected in up to 90% of murine APL cells by flow cytometry. Collectively, these results indicate that JAG1 overexpression is a common event in both human and murine APL pathogenesis.Hypothesizing that JAG1 overexpression could lead to Notch signaling in APL cells, we measured Notch signaling in human and murine APL cells. The Notch receptors Notch-1 and Notch-2, the components of the gamma secretase complex, and the transcriptional coactivators of Notch are highly expressed in nearly all human APL samples, suggesting that APL cells are capable of Notch signaling. Using bioinformatic approaches, we found that three previously defined activated Notch signatures were significantly enriched in APL cells (FDR=0.00) and also in PR9 and NB4 cells. Protein levels of cleaved Notch-1 increased in induced PR9 cells with the same kinetics as JAG1 protein, peaking at 16 hours post-induction. In addition, flow cytometry demonstrated the presence of cleaved Notch protein in PR9 and NB4 cells, and both primary and immortalized murine APL samples. To define the consequences of inhibiting Notch signaling in APL cells, we cultured primary murine APL cells in the presence of gamma secretase inhibitors; colony formation in 6/9 tumors was significantly inhibited (p<0.05). Growth of primary murine marrow and spleen cells was unaffected under identical culture conditions.Finally, we investigated the role of Notch signaling in APL pathogenesis in young mCG-PR mice prior to the development of leukemia. Marrow derived progenitors from PML-RARA mice replate serially in methylcellulose cultures, in contrast to wildtype marrow cells, which do not replate. In contrast to primary tumor cells, which were variably dependent on Notch signaling, preleukemic cells were uniformly dependent on Notch signaling for replating activity. In the presence of gamma secretase inhibitors, the replating phenotype is significantly decreased (p<0.01). In contrast, colony formation by wildtype cells is unaffected by inhibition of Notch signaling. Similarly, when Notch signaling in primary marrow cells was genetically inhibited by retroviral transduction of a dominant negative mastermind construct (DNMAML), colony formation by PML-RARA cells was significantly decreased at the first, second and third replatings compared to GFP control cells (p<0.01). Again, wildtype marrow colony formation was unaffected by DNMAML transduction. Importantly, wild type marrow cells transduced with a JAG1 expressing retrovirus did not acquire a replating phenotype, nor did animals transplanted with JAG1 expressing marrow develop leukemia. In conclusion, we have demonstrated overexpression of JAG1 and activation of Notch signaling in human and murine APL cells, and have shown that Notch signaling may play a role in the early pathogenesis of APL. Collectively, these results support a model in which PML-RARA induces the self-renewal of hematopoietic progenitors via a Notch signaling pathway, which may be a critical early step in APL pathogenesis. Disclosures:No relevant conflicts of interest to declare.

Myeloma Cell Survival and Importance of Crosstalk Between Notch1-Jagged2 and CD28-B7 Pathways In Dendritic Cells

AbstractAbstract 1741Multiple myeloma is a neoplasm of bone marrow resident plasma cells characterized by critical interactions between myeloma cells and bone marrow stromal cells. This interaction leads to production of IL-6, an important factor in myeloma cell biology. However, the molecular and cellular components involved in myeloma induced IL-6 production remain largely uncharacterized. While at the cellular level, dendritic cells (DC)-expressing CD80/CD86 (collectively called B7, ligands with short cytoplasmic tails and signaling partners of CD28 expressed on myeloma cells) - in the bone marrow microenvironment have been implicated as being an important component, at the molecular level the CD28-B7 and Notch1-Jagged2 pathways were separately implicated by us (in DC) and others in myeloma induced IL-6 production. Although Notch signaling leading to IL-6 production in DC is well understood, the mechanism of “backsignaling” via B7 is largely uncharacterized.To better understand downstream B7 signaling leading to IL-6 production, DC were stimulated with CD28-Ig (a soluble form of CD28 which mimicks myeloma cell-bound CD28) in the presence or absence of an inhibitor of Notch signaling, gamma secretase inhibitor (GSI). DC treated with CD28-Ig alone produced significantly (p< 0.001) higher levels of IL-6 when compared to DC treated with CD28-Ig and GSI. GSI specifically targeted Notch signaling as observed by decreased expression of Notch gene targets: Hes-1 (2 fold decrease) and Deltex-4 (4 fold decrease). Also, decreased IL-6 levels in presence of GSI were not due to the decrease in B7 expression on DC. To specifically implicate the importance of Notch1 and Jagged2, we blocked Notch1 signaling using blocking antibodies and observed a similar decrease in IL-6 production upon blocking Notch1 signaling. Our results suggest that CD28 mediated IL-6 production is dependent on Notch1 signaling and crosstalk between the Notch1-Jagged2 and CD28-B7 pathways leads to IL-6 production by DC. The model of crosstalk between CD28-B7 and Notch1-Jagged2 pathways was also observed in murine bone marrow derived dendritic cells (BMDC), where a significant (p<0.001) down regulation of IL-6 was observed upon blocking Notch signaling.One possible mechanism of crosstalk involves direct effect of B7 crosslinking by CD28-Ig on Notch expression/signaling leading to increase in IL-6 production. We tested for this possibility in DC and found no significant change in Notch expression/signaling. We thus hypothesized that the mechanism of crosstalk involves molecules downstream to Notch and/or B7. Notch signaling has been reported to be involved in the regulation of PTEN (a negative regulator of the PI3K/Akt pathway). Previous studies have also shown the importance of FoxO3a-a transcription factor tightly regulated by Akt- in regulating IL-6 production in BMDC upon B7 crosslinking. We therefore tested the possible involvement of PTEN (molecule downstream of Notch signaling), Akt and FoxO3a (molecules downstream of B7) in crosstalk between the two pathways aforementioned by testing the effect of GSI on their regulation at the protein level. We observed an approximate 2 fold decrease in phospho-PTEN/PTEN ratio in DC treated with GSI and remained so even after B7 crosslinking at an early time point (15 min. post CD28-Ig treatment.) Further, phospho-Akt/Akt ratio decreased by 1.6 fold in DC treated with both GSI and CD28-Ig compared to CD28-Ig alone at 30 min. We therefore hypothesize a model of crosstalk involving Notch mediated regulation of PTEN leading to IL-6 production via regulation of Akt and possibly FoxO3a upon B7 crosslinking. Interestingly enough “backsignaling” via B7 in myeloma-induced IL-6 production seems to involve molecules well characterized in CD28 signaling of T-cells. Targeting IL-6 induced by crosstalk between these two pathways prompts not only clinical evaluation to improve MM patient outcome but also extends to advancing knowledge in T-cell and normal plasma cell biology as well. Disclosures:No relevant conflicts of interest to declare.

MUC1 is a substrate for γ‐secretase

Understanding the underlying mechanisms by which a normal cell avoids the oncogenic potential of MUC1 signaling requires further definition of the pathways by which the MUC1 cytoplasmic tail is processed in both normal and tumor-derived cells. In the present study we describe the processing pathway initiated by TACE/ADAM17 cleavage of MUC1. Utilizing the human uterine epithelial cell line, HES, derived from normal endometrium, we show that endogenous full length MUC1 undergoes regulated intramembranous proteolysis mediated by presenillin-dependent gamma-secretase. Cytokine-stimulated HES cells exposed to gamma-secretase inhibitors accumulated a membrane-associated 15 kDa fragment of the MUC1 C-terminal subunit (CTF15). Inhibitors of TACE/ADAM17-mediated shedding inhibited accumulation of MUC1-CTF15 and MUC1 ectodomain release to a similar extent consistent with MUC1-CTF15 being a product of TACE/ADAM17 action. Reduction of catalytically active gamma-secretase complex by nicastrin siRNA treatment also resulted in CTF15 accumulation. Furthermore, mature nicastrin, the substrate receptor for gamma-secretase, co-immunoprecipitated with CTF15 in the presence of gamma-secretase inhibitors indicating the formation of CTF15: nicastrin complexes. MUC1-CTF15 accumulation in response to gamma-secretase inhibition was demonstrated in both normal and tumor-derived cells from humans and mice indicating that this processing pathway exists in many cell contexts. We did not detect products of MUC1 cleavage by gamma-secretase in the presence of various proteasomal inhibitors indicating that subsequent degradation is either non-proteasomal or extremely efficient. We suggest that this efficient pathway attenuates potential signaling mediated by cytoplasmic tail fragments.

Notch Signaling Is Inactive but Inducible in Human Embryonic Stem Cells

The NOTCH signaling pathway performs a wide range of critical functions in a number of different cell types during development and differentiation. The role of NOTCH signals in human embryonic stem cells (hESCs) has not been tested. We measured the activity of canonical NOTCH signaling in undifferentiated embryonic stem (ES) cells and tested the requirement for NOTCH activity in hESC self-renewal or differentiation by growing hESCs in the presence of gamma-secretase inhibitors. Our results suggest that NOTCH signaling is not required for the propagation of undifferentiated human ES cells but instead is required for the maintenance of the differentiating cell types that accumulate in human ES cell cultures. Our studies suggest that NOTCH signaling is not required in human embryonic differentiation until the formation of extraembryonic, germ layer, or tissue-specific stem cells and progenitors.

Efficient Inhibition of Notch3 and Notch4 Family Members In Vivo by a Dominant Negative Mutant of Mastermind.

The four mammalian Notch receptors play multiple roles in the hematolymphoid system. Notch1 is critical for the generation of hematopoietic stem cells (HSCs) during embryonic development and for commitment to the T cell lineage, while Notch2 regulates marginal zone B cell development. In addition, important roles for Notch are starting to be identified in peripheral T cells, in which all four Notch family members are upregulated upon T cell activation. Canonical Notch signaling involves proteolytic cleavage of Notch receptors and translocation of intracellular Notch (ICN) to the nucleus where it interacts with the transcription factor CSL/RBP-J and recruits Mastermind-like proteins (MAMLs) for transcriptional activation. By inhibiting Notch1 and Notch2-dependent developmental decisions with a dominant negative mutant of MAML1 (DNMAML1), we have previously demonstrated that MAMLs critically regulate Notch signaling in vivo (Maillard et al., Blood 2004). However, it is unclear if Notch3 and Notch4 are equally dependent on the activity of MAMLs in vivo, and if DNMAML1 can thus be used as a true pan-Notch inhibitor.To address this question, we used a modified fetal thymic organ culture (FTOC) system and Notch-mediated T lineage commitment as a readout for Notch function. Fetal liver cells (FLCs) were transduced with retroviruses expressing ICN1, ICN3 or ICN4 and a retrovirus expressing DNMAML1. Irradiated fetal thymic lobes were reconstituted with transduced FLCs and cultured in the presence of gamma secretase inhibitors to inhibit the activation of endogenous Notch receptors. In this manner the effect of a specific ICN could be evaluated in the absence of endogenous Notch signals. Gamma secretase inhibitors resulted in a profound block in T cell development and in the generation of intrathymic B cells, consistent with Notch1 inhibition. ICN1, ICN3 and ICN4 were all able to rescue commitment to the T cell lineage and to block B cell development, indicating that each ICN can deliver appropriate signals for T lineage commitment in the absence of endogenous Notch1 signaling. In cells doubly transduced with specific ICNs and DNMAML1, the rescuing effect of ICN1, ICN3 or ICN4 was blocked, leading to the generation of B cells. In this competitive situation, DNMAML1 only partially inhibited ICN1, the most potent of all four Notch family members, but it was able to completely inhibit ICN3 and ICN4. We conclude that DNMAML1 can efficiently inhibit Notch3 and Notch4 in vivo. The results validate the use of DNMAML1 as a bona fide pan-Notch inhibitor in multiple settings, including studying the role of Notch in HSCs and peripheral T cells.

Generation of C-terminally truncated amyloid-beta peptides is dependent on gamma-secretase activity.

Aberrant production of amyloid-beta peptides by processing of the beta-amyloid precursor protein leads to the formation of characteristic extracellular protein deposits which are thought to be the cause of Alzheimer's disease. Therefore, inhibiting the key enzymes responsible for amyloid-beta peptide generation, beta- and gamma-secretase may offer an opportunity to intervene with the progression of the disease. In human brain and cell culture systems a heterogeneous population of amyloid-beta peptides with various truncations is detected and at present, it is unclear how they are produced. We have used a combination of surface enhanced laser desorption/ionization time-of-flight mass spectrometry (SELDI-TOF MS) and a specific inhibitor of gamma-secretase to investigate whether the production of all amyloid-beta peptide species requires the action of gamma-secretase. Using this approach, we demonstrate that the production of all truncated amyloid-beta peptides except those released by the action of the nonamyloidogenic alpha-secretase enzyme or potentially beta-site betaAPP cleaving enzyme 2 depends on gamma-secretase activity. This indicates that none of these peptides are generated by a separate enzyme entity and a specific inhibitor of the gamma-secretase enzyme should havethe potential to block the generation of all amyloidogenicpeptides. Furthermore in the presence of gamma-secretase inhibitors, the observation of increased cleavage of the membrane-bound betaAPP C-terminal fragment C99 by alpha-secretase suggests that during its trafficking C99 encounters compartments in which alpha-secretase activity resides.